Belching dinosaurs may have helped keep their world warm

The gut bacteria of giant plant-eaters made a lot of the greenhouse gas methane.

The world inhabited by dinosaurs was typically a hot one, with high levels of greenhouse gasses, lots of water vapor, and no permanent ice sheets. And, according to a new estimate published in the journal Current Biology, the dinosaurs themselves may have contributed to their hothouse conditions.

(Note to the editors of Current BIology: I'm not sure that the Jurassic really qualifies as "current.")

A team of British researchers has put together various estimates of the features of the Jurassic's large herbivores, such as population density, typical body mass, and so on. Combined with an estimate of how much methane is emitted by a typical herbivore, these numbers suggest that the dinosaurs were pouring out enough methane to help the greenhouse effect keep the Earth nice and toasty.

Methane is a potent greenhouse gas, being capable of driving far more heat retention than an equivalent volume of carbon dioxide. And, even when it breaks down, it does so by being converted to carbon dioxide and water vapor, both of which are also greenhouse gasses. So, a little methane can go a long way towards keeping the planet hot.

For many bacteria, methane is simply the end-point of their metabolism (in the same way that water and CO2 are for us). In conditions where oxygen is scarce or absent, these organisms pump out methane as a byproduct of the reactions that keep them supplied with energy. One such location is within the guts of plant-eating animals. There, the methanogens help break down the plant material, helping the cow digest it. In return, they get a meal, resulting in the release of methane. This makes its way into the atmosphere exactly as you suspect it would.

For a single animal, this is no big deal. But there are a lot of plant eaters out there. The US cattle industry is large enough that it contributes 20 percent of the nation's annual methane emissions—and the US has less than 10 percent of the world's cows. Overall, cows account for something like 75 Teragrams of methane emissions annually (technically, this is known as "a lot").

By modern standards, a typical cow seems like a fairly large animal, but they're positively tiny compared to some of the large plant-eating dinosaurs. A medium sized sauropod (the Apatosaurus louise is the authors' choice) weighed in at something like 20,000kg. And, all estimates are that these animals could live at a pretty high population density, possibly because the high temperatures, CO2, and water vapor allowed plants that were adapted to these conditions to flourish. Add in the fact that a lot of the world could probably constitute habitat (again, there were no ice sheets at the time), and you have a whole lot of dinosaurs providing a good habitat for methane-producing bacteria.

Since these dinosaurs were probably not warm-blooded, the environment they created for their bacteria probably wasn't as favorable as that provided by a cow. But even with a reduced amount of methane produced as a function of body weight, even a single animal could produce an estimated 2,675 liters of methane a day. Multiply that by the population estimates, and you get a staggering 520 Teragrams of methane a year. That's roughly equivalent to the total methane emissions for our entire planet at the moment. Even if the authors' estimates are high by a factor of two, it's still more than the preindustrial methane emissions.

Now, there are a lot of estimations involved here: population density, area of available habitat, emissions per animal. But the authors pretty reasonably state that, even if all of these are wrong, the dinosaurs probably still made a significant contribution to the planet's methane cycle. If any of these are underestimates, the collective outpouring of their belches could have been a major influence on the climate.

It doesn't seem likely to have any practical applications, but the work is a nice reminder of just how much biology can shape its own habitat. And, if nothing else, I suspect this will be the only time I ever time I ever read a paper containing a bar chart with items labeled "Modern," "Cows," and "Sauropods."